N-oxyl compounds, process for the preparation thereof, and process for inhibiting the polymerization of vinyl monomers with the same

ABSTRACT

Disclosed are novel N-oxyl compounds of the following formula (1).                    
     wherein n is an integer of 1 to 18; R 1  and R 2  are each hydrogen or methyl, but at least one of them is hydrogen; R 3 , R 4 , R 5  and R 6  are each a straight-chain or branched alkyl group; and R 7  is hydrogen or (meth)acryloyl. When these compounds are added to vinyl monomers such as α,β-unsaturated carboxylic acids and esters thereof, they exhibit a satisfactory polymerization-inhibiting effect even at low contents and even at elevated temperatures.

This application is a 371 of PCT/JP98/05085 filed Nov. 12, 1998.

TECHNICAL FIELD

This invention relates to novel N-oxyl compounds, mixtures of suchN-oxyl compounds, a process for preparing them, and a method forinhibiting the polymerization of vinyl monomers, such as α,β-unsaturatedcarboxylic acids and esters thereof, by using them.

BACKGROUND ART

α,β-Unsaturated carboxylic acids and esters thereof, which are typicalof vinyl monomers, are compounds having a wide variety of uses asmonomers for the production of various polymers and copolymers. However,since they have the property of being very easily polymerized,polymerization troubles due to causative factors such as heat and lightmay frequently occur during the course of their production, storage,transport and the like. Especially in the production of α,β-unsaturatedcarboxylic acids and esters thereof, they are exposed to hightemperatures above 100° C. and hence tend to cause the trouble offorming a popcorn polymer in the liquid and vapor phases.

In order to prevent the above-described polymerization troublesoccurring in α,β-unsaturated carboxylic acids and esters thereof,attempts have been made to inhibit their polymerization by using variouspolymerization inhibitors. Such polymerization inhibitors include, forexample, heteroaromatic compounds such as phenothiazine; phenols such ashydroquinone and hydroquinone monomethyl ether; and aromatic amines suchas N,N′-di-2-naphthyl-p-phenylenediamine [Nonflex F (trade name),manufactured by Seiko Chemical Co., Ltd.; hereinafter abbreviated as“AF”], N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine [Nocrak 6C(trade name), manufactured by Ouchishinko Chemical Industrial Co., Ltd.;hereinafter abbreviated as “6C”] and N,N′-diphenyl-p-phenylenediamine[Nonflex H (trade name), manufactured by Seiko Chemical Co., Ltd.].Moreover, Japanese Patent Laid-Open Nos. 320205/'93 and 320217/'93disclose a method using an N-oxyl compound of the following formula (2)alone or in combination with other polymerization inhibitors.

wherein R⁸ represents hydrogen, an alkyl group or an acyl group.

However, under high-temperature conditions employed for the purpose ofreaction and purification by distillation in processes for theproduction of α,β-unsaturated carboxylic acids and esters thereof,conventional polymerization inhibitors fail to exhibit a satisfactorypolymerization-inhibiting effect. Consequently, these polymerizationinhibitors need to be added in large amounts and hence have a problemfrom the viewpoint of industrial use. Moreover, although the N-oxylcompounds of formula (2) in which R⁸ is hydrogen or an alkyl group havean excellent polymerization-inhibiting ability even when used in smallamounts, the actually known compounds of this type are limited to thosein which R⁸ is hydrogen, methyl or ethyl. These compounds haverelatively low boiling points or sublimation temperatures which areclose to the boiling points of α,β-unsaturated carboxylic acids andesters thereof, and hence involve the problem that, during thepurification of α,β-unsaturated carboxylic acids and esters thereof bydistillation, they are distilled out together with the product to causea coloration thereof. On the other hand, the N-oxyl compounds of formula(2) in which R⁸ is an acyl group have sufficiently high boiling pointsor sublimation temperatures. However, during use, they may be convertedto compounds having low boiling points or sublimation temperatures andmay eventually be distilled out together with the product. As an exampleof this phenomenon, when an N-oxyl compound of formula (2) is used as apolymerization inhibitor for the ester exchange reaction between methyl(meth)acrylate and an alcohol, the polymerization inhibitor itself mayundergo an ester exchange reaction to form a compound containing a(meth)acryloyl group as R⁸ and hence having a low boiling point orsublimation temperature, and may eventually be distilled out togetherwith the product to cause a coloration thereof.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide novel N-oxyl compoundsand a process for preparing them.

Another object of the present invention is to provide a polymerizationinhibition method which, when employed in processes for the productionof vinyl monomers such as α, β-unsaturated carboxylic acids and estersthereof, causes little coloration of the product and does not require alarge amount of polymerization inhibitor.

That is, the present invention relates to an N-oxyl compound of thefollowing formula (1).

wherein n is an integer of 1 to 18; R¹ and R² are each hydrogen ormethyl, but at least one of them is hydrogen; R³, R⁴, R⁵ and R⁶ are eacha straight-chain or branched alkyl group; and R⁷ is hydrogen or(meth)acryloyl.

Moreover, the present invention also relates to a process for thepreparation of an N-oxyl compound of formula (1) which comprises thesteps of effecting the addition of ethylene oxide and/or propylene oxideto a 4-hydroxy-2,2,6,6-tetraalkylpiperidine-N-oxyl, and optionallyesterifying the resulting product with a (meth)acryloyl-containingcompound.

Furthermore, the present invention also relates to a polymerizationinhibition method wherein an N-oxyl compound of formula (1) or a mixtureof such N-oxyl compounds is used as a polymerization inhibitor for vinylmonomers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphs showing the results of analysis byhigh-performance liquid chromatography for the N-oxyl compounds (IB andIE) of the present invention which were obtained in Examples 1 and 2,respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

The N-oxyl compounds of the present invention are compounds representedby formula (1). In this formula, each of R³, R⁴, R⁵ and R⁶ is astraight-chain or branched alkyl group. However, each of R³, R⁴, R⁵ andR⁶ is preferably a straight-chain alkyl group of 1 to 4 carbon atoms andmore preferably methyl.

Although the N-oxyl compounds of formula (1) in accordance with thepresent invention may be obtained in the form of a pure compound inwhich n is an integer of 1 to 18, they are usually obtained as a mixtureof two or more N-oxyl compounds having different values of n. In thiscase, the average value of n per mole of the N-oxyl compounds(hereinafter referred to as the average number of moles added) issuitably in the range of 1 to 14. If the average number of moles addedis greater than 14, the molecular weight becomes so great that theweight required to provide the same number of moles will be undulyincreased. Moreover, the boiling point or sublimation temperature of themixture of N-oxyl compounds is raised to such an extent that it cannotbe easily used as a polymerization inhibitor. The preferred range of theaverage number of moles added is from 2 to 10.

The N-oxyl compounds of the present invention in which R⁷ in formula (1)is hydrogen may be synthesized by effecting the addition reaction ofethylene oxide and/or propylene oxide, for example, to4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl.

According to one exemplary procedure for effecting the addition reactionof ethylene oxide, a four-necked flask fitted with a stirrer, athermometer and a brine condenser is charged with4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, a solvent and acatalyst. Then, ethylene oxide is introduced into the system by blowingit in at such a rate that it is not refluxed in the lower part of thebrine condenser. After a predetermined amount of ethylene oxide has beenblown in, the stirring is continued until the disappearance of4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl used as a startingmaterial is confirmed. After the reaction is stopped, the reactionmixture is stirred under reduced pressure to remove any unreactedethylene oxide. Consequently, an N-oxyl compound of formula (1), or amixture of such N-oxyl compounds, is obtained as residue. Similarly, theaddition reaction of propylene oxide may be carried out by controllingthe dropping rate of propylene oxide so that it is not refluxed in thelower part of the brine condenser.

The solvents which can be used for the addition reaction include, forexample, halogenated hydrocarbons such as methylene chloride,chloroform, carbon tetrachloride and chlorobenzene; aromatichydrocarbons such as benzene, toluene, xylene, cumene and ethylbenzene;and ether solvents such as diethyl ether, diisopropyl ether,tetrahydrofuran, tetrahydropyran and dioxane. Among others, aromatichydrocarbons are preferably used.

As the catalyst for the addition reaction of ethylene oxide and/orpropylene oxide, there may be used any of the common polymerizationcatalysts for ethylene oxide and/or propylene oxide. For example, alkalihydroxides, iron(III) chloride and tin(IV) chloride are preferably used.Among them, iron(III) chloride is especially preferred.

In carrying out the addition reaction of ethylene oxide and/or propyleneoxide, it is necessary to add ethylene oxide and/or propylene oxidewhile maintaining the reaction mixture at such a temperature that theyare not refluxed. Specifically, the reaction mixture is maintained at atemperature of −10 to 40° C. and preferably 0 to 30° C. After completionof the addition of ethylene oxide and/or propylene oxide, the reactionis preferably carried out at an elevated temperature of 20 to 50° C.After completion of the reaction, degassing is also preferably carriedout at a temperature of 20 to 50° C.

The N-oxyl compounds of the present invention in which R⁷ in formula (1)is (meth)acryloyl may be synthesized by effecting the addition reactionof ethylene oxide and/or propylene oxide, for example, to4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, and subsequentlysubjecting the resulting product to esterification with (meth)acrylicacid or ester exchange with a (meth)acrylate.

An N-oxyl compound having a given value of n may be obtained bycontrolling the amount of ethylene oxide and/or propylene oxide added.However, the N-oxyl compound obtained in this manner usually comprises amixture of two or more N-oxyl compounds having different values of n. Ifnecessary, therefore, pure N-oxyl compounds having a definite value of ncan be isolated by purifying the mixture according to such techniques ascolumn chromatography and distillation. For the mixture of N-oxylcompounds obtained by the addition reaction, the average number of molesadded can be determined by dividing the number of moles of ethyleneoxide and/or propylene oxide added by the number of moles of4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl converted to the adduct.

The N-oxyl compounds of formula (1) in accordance with the presentinvention can be used as polymerization inhibitors for preventing thepolymerization of vinyl monomers. These polymerization inhibitors havehigher boiling points or sublimation temperatures as n is increased.Accordingly, in order to reduce the amount of polymerization inhibitorcontaminated into the product in the distillation process, it ispreferable to select a polymerization inhibitor having a boiling pointor sublimation temperature which is substantially different from theboiling point of the vinyl monomer as the product.

As compared with aromatic amine compounds conventionally used aspolymerization inhibitors, the N-oxyl compounds of formula (1) arecharacterized in that they have excellent solubility in α,β-unsaturatedcarboxylic acids and esters thereof and in that the equipment and tankscan be easily cleaned after use. For this reason, they are veryfavorable for industrial use. When, among the N-oxyl compounds offormula (1), those in which R⁷ is hydrogen are used as polymerizationinhibitors in an ester exchange reaction process using methyl(meth)acrylate as a starting material, part or all of them may beconverted to compounds in which R⁷ is (meth)acryloyl. However, there isno problem from a practical point of view because theirpolymerization-inhibiting ability is comparable to that of thecorresponding compounds in which R⁷ is hydrogen and they do not tend tobe distilled out into the product.

The polymerization inhibition method of the present invention can beapplied not only to the purpose of polymerization inhibition inprocesses for the production of vinyl monomers, preferablyα,β-unsaturated carboxylic acids and esters thereof, but also to thepurpose of polymerization inhibition during storage and transportthereof.

In the polymerization inhibition method of the present invention, atleast an N-oxyl compound of formula (1) or a mixture of two or moreN-oxyl compounds having different values of n in formula (1)(hereinafter referred to as the mixture of such N-oxyl compounds) isused as a polymerization inhibitor. However, it is a matter of coursethat other general-purpose polymerization inhibitors may be used incombination therewith. The combined use of other polymerizationinhibitors may produce a more excellent polymerization-inhibiting effectowing to the synergistic action of those polymerization inhibitors.

The amount in which the N-oxyl compound of formula (1) or the mixture ofsuch N-oxyl compounds is added may vary according to the process andconditions employed. However, when they are used alone, the amount addedis generally in the range of about 1 to 1,000 ppm based on the vinylmonomer whose polymerization is to be inhibited. When they are used incombination with other polymerization inhibitors, it is generally in therange of about 0.5 to 1,000 ppm.

All of the N-oxyl compounds of formula (1) and the mixtures of suchN-oxyl compounds have a polymerization-inhibiting effect on vinylmonomers. Accordingly, when these polymerization inhibitors are used, asuitable polymerization inhibitor may be selected with consideration forthe boiling point of the vinyl monomer, the operating conditions or thelike.

In the practice of the present invention, the N-oxyl compound of formula(1), or the mixture of such N-oxyl compounds, used as a polymerizationinhibitor is added to a vinyl monomer either as such or in the form of asolution. Where it is desired to prevent the polymerization of a vinylmonomer within a distillation column, it is common practice to dissolvethe polymerization inhibitor, for example, in a substance contained inthe distillation system and supply this solution to the top or middle ofthe distillation column. In this case, if a mixture of two or moreN-oxyl compounds having different value of n in formula (1) is used,these compounds have different boiling points and, therefore, are widelydistributed within the distillation column as contrasted with a purecompound which is present only in some part of the distillation column.Accordingly, the use of a mixture represents a preferred embodiment inthat a polymerization-inhibiting effect on the vinyl monomer isexhibited over a great part of the distillation column.

In the polymerization inhibition method of the present invention, thevinyl monomer containing an N-oxyl compound of the above formula (1) ora mixture of such N-oxyl compounds as a polymerization inhibitor mayadditionally contain molecular oxygen or air, if necessary, in order toachieve a further enhancement in polymerization-inhibiting effect. Thismay readily be accomplished by such techniques as air bubbling.

The polymerization inhibition method of the present invention can beapplied to vinyl monomers in general, and it can be applied not only toa single vinyl monomer but also to a mixture of two or more vinylmonomers. Moreover, an excellent effect is achieved especially when thepolymerization inhibition method of the present invention is applied toα,β-unsaturated carboxylic acids and esters thereof. Useful vinylmonomers include ethylene, propylene, butadiene, styrene, vinyl chlorideand the like. Useful α,β-unsaturated carboxylic acids and esters thereofinclude, for example, acrylic acid, methyl acrylate, ethyl acrylate,n-butyl acrylate, i-butyl acrylate, stearyl acrylate, 2-ethylhexylacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,4-hydroxybutyl acrylate, ethylene glycol diacrylate, glycidyl acrylate,2-ethoxyethyl acrylate, tetrahydrofurfuryl acrylate, cyclohexylacrylate, benzyl acrylate, allyl acrylate, t-butyl acrylate,1,6-hexanediol diacrylate, dimethylaminoethyl acrylate, methacrylicacid, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate,i-butyl methacrylate, stearyl methacrylate, 2-ethylhexyl methacrylate,2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, ethyleneglycol dimethacrylate, glycidyl methacrylate, 2-ethoxyethylmethacrylate, cyclohexyl methacrylate, benzyl methacrylate, allylmethacrylate, t-butyl methacrylate, dimethylaminoethyl methacrylate,phenyl methacrylate, crotonic acid, methyl crotonate, ethyl crotonate,itaconic acid, dimethyl itaconate, methyl α-hydroxyethylacrylate andethyl α-hydroxyethylacrylate. It is a matter of course that the presentinvention is not limited thereto.

The present invention is more specifically explained with reference tothe following examples and comparative examples. The polymerizationinhibitors used therein are designated by the abbreviations shown inTables 1 and 2. In all of the polymerization inhibitors of formula (1)in which R¹, R² and the average number of moles added (n) are varied asshown in Table 1, R³, R⁴, R⁵ and R⁶ are CH₃ and R⁷ is H.

TABLE 1 n, R¹ and R² in compound of general formula (1) Abbreviation nR¹ R² IA 2 H H IB 6 H H IC 10 H H ID 0 — — IE 3 One of R¹ and R² is Hand the other is CH₃ IF 6 One of R¹ and R² is H and the other is CH₃ IG10 One of R¹ and R² is H and the other is CH₃

TABLE 2 Abbreviation Structural formula of polymerization inhibitor HQ

MEHQ

PZ

AF

6C

IH

MTX

The α,β-unsaturated carboxylic acids and esters thereof which were usedtherein are designated by the following abbreviations.

CHMA: Cyclohexyl methacrylate

EDMA: Ethylene glycol dimethacrylate

BMA: n-Butyl methacrylate

BZMA: Benzyl methacrylate

EHMA: 2-Ethylhexyl methacrylate

AMA: Allyl methacrylate

HEMA: 2-Hydroxyethyl methacrylate

GMA: Glycidyl methacrylate

HEA: 2-Hydroxyethyl acrylate

MA: Methyl acrylate

MMA: Methyl methacrylate

AA: Acrylic acid

MAA: Methacrylic acid

EXAMPLE 1

A four-necked flask fitted with a stirrer, a thermometer and a brinecondenser was charged with4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl as a starting material,toluene and iron(III) chloride. While this mixture was being stirred,ethylene oxide was blown thereinto to effect an addition reaction at areaction temperature of 10-15° C. The blowing rate of ethylene oxide wassuch that it was not refluxed in the lower part of the brine condenser.After ethylene oxide was blown in until its molar amount added becameequal to six times that of4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl charged, the stirring wascontinued. After the disappearance of4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl was confirmed throughanalysis by high-performance liquid chromatography, the reaction wasstopped. Thereafter, the reaction mixture was stirred under reducedpressure to remove any unreacted ethylene oxide, and then washed withwater to remove any 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl andthe catalyst. Finally, the toluene was removed under reduced pressure toobtain IB shown in Table 1. The results of analysis of IB byhigh-performance liquid chromatography are shown in FIG. 1. Theconditions employed for analysis by high-performance liquidchromatography were as follows.

Measuring Conditions of High-performance Liquid Chromatography

Apparatus: Shimadzu LC-6A

Column: Inertsil ODS-80A, 4.6 mmφ×250 mm

Mobile phase: CH₃CN/H₂O/H₃PO₄=400/600/1

Flow velocity: 1.0 ml/min

Temperature: 40° C.

Detection: UV 240 nm (ABS 0.02)

Moreover, IA and IC were prepared in the same manner as described forIB, except that the amount of ethylene oxide blown in was varied.

EXAMPLE 2

A four-necked flask fitted with a stirrer, a thermometer and a brinecondenser was charged with4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl as a starting material,toluene and iron(III) chloride. While this mixture was being stirred,propylene oxide was dropped thereinto to effect an addition reaction ata reaction temperature of 15-20° C. The dropping rate of propylene oxidewas such that it was not refluxed in the lower part of the brinecondenser. After propylene oxide was dropped until its molar amountadded became equal to three times that of4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl charged, the stirring wascontinued. After the disappearance of4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl was confirmed throughanalysis by high-performance liquid chromatography, the reaction wasstopped. Thereafter, the reaction mixture was stirred under reducedpressure to remove any unreacted propylene oxide, and then washed withwater to remove any 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl andthe catalyst. Finally, the toluene was removed under reduced pressure toobtain IE shown in Table 1. The results of analysis of IE byhigh-performance liquid chromatography are shown in FIG. 2. Theconditions employed for analysis by high-performance liquidchromatography were the same as for IA.

Moreover, IF and IG were prepared in the same manner as described forIE, except that the amount of propylene oxide blown in was varied.

EXAMPLES 3-8 AND COMPARATIVE EXAMPLES 1-5

Test solutions were prepared by adding 5 ppm of each polymerizationinhibitor to the various (meth)acrylic esters shown in Table 3 and fromwhich any polymerization inhibitor had previously been removed bydistillation. Then, 15 g each of the aforesaid test solutions werepoured into 25 ml ampules. These ampules were sealed with siliconestoppers and immersed in an oil bath kept at 70° C. for methyl acrylate(MA), 90° C. for methyl methacrylate (MMA), or 120° C. for other(meth)acrylic esters. While this oil bath was being shaken on a shaker,the ampules were visually observed to confirm the time at which popcornformation, gelation or an increase in solution viscosity occurred. Thus,the polymerization initiation time (in minutes) of the (meth)acrylicester in each ampule was determined. The results thus obtained are alsoshown in Table 3.

TABLE 3 Polymerization Polymerization initiation time (min.) No.inhibitor CHMA EDMA BMA AMA HEMA GMA EHMA BZMA HEA MA MMA Example 3 IA849 993 1001 >1100 713 815 1054 1035 630 >1100 >1100 Example 4 IB 519598 608 681 449 503 666 639 674 >1100 880 Example 5 IC 376 420 451 505328 340 490 421 380 758 667 Example 6 IE 653 751 778 876 553 610 838 767495 >1100 >1100 Example 7 IF 433 500 525 588 379 439 560 530 357 889 734Example 8 IG 304 333 365 408 258 288 390 338 274 590 503 Comparative PZ57 31 50 42 40 48 70 60 35 126 112 Example 1 Comparative HQ 60 31 52 4341 51 75 64 41 124 120 Example 2 Comparative MEHQ 12 5 10 7 9 10 13 1311 47 38 Example 3 Comparative AF 43 21 37 30 31 35 54 46 30 120 106Example 4 Comparative 6C 23 5 19 15 13 15 30 27 14 63 60 Example 5

EXAMPLES 9-14 AND COMPARATIVE EXAMPLES 6-10

Test solutions were prepared by adding 5 ppm of each of thepolymerization inhibitors shown in Table 4 to (meth)acrylic acid fromwhich any polymerization inhibitor had previously been removed bydistillation. Then, 15 g each of the aforesaid test solutions werepoured into 25 ml ampules. These ampules were sealed with siliconestoppers and immersed in an oil bath kept at 100° C. While this oil bathwas being shaken on a shaker, the ampules were visually observed toconfirm the time at which popcorn formation, gelation or an increase insolution viscosity occurred. Thus, the polymerization initiation time(in minutes) of (meth)acrylic acid in each ampule was determined. Theresults thus obtained are also shown in Table 4.

TABLE 4 Polymerization Polymerization initiation time (min.) Example No.inhibitor Acrylic acid Methacrylic acid Example 9 IA 77 126 Example 10IB 40 79 Example 11 IC 28 51 Example 12 IE 58 94 Example 13 IF 36 64Example 14 IG 25 43 Comparative Pz 8 16 Example 6 Comparative HQ 1 2Example 7 Comparative MEHQ 2 4 Example 8 Comparative AF 7 14 Example 9Comparative 6C 3 6 Example 10

It can be seen from Tables 3 and 4 that, when the N-oxyl compounds ofthe above formula (1) were used as polymerization inhibitors, thepolymerization initiation time of α,β-unsaturated carboxylic acids andesters thereof was markedly prolonged to exhibit a more excellentpolymerization-inhibiting effect, as compared with the cases in whichconventional polymerization inhibitors such as phenothiazine,hydroquinone, hydroquinone monomethyl ether, AF, and 6C were used.

EXAMPLES 15-19 AND COMPARATIVE EXAMPLES 11-14

Test solutions were prepared by adding each of the polymerizationinhibitors shown in Table 5 to ethylene glycol dimethacrylate from whichany polymerization inhibitor had previously been removed bydistillation. Then, 15 g each of the aforesaid test solutions werepoured into 25 ml ampules. These ampules were sealed with siliconestoppers and immersed in an oil bath kept at 120° C. While this oil bathwas being shaken on a shaker, the ampules were visually observed todetermine the polymerization initiation time (in minutes) of ethyleneglycol dimethacrylate in each ampule. The results thus obtained are alsoshown in Table 5.

TABLE 5 Polymerization Amount added Polymerization No. inhibitor (ppm)initiation time (min.) Example 15 IB 3 159 Example 16 IB 3 195 MEHQ 10Example 17 IB 3 228 HQ 10 Example 18 IB 3 210 AF 10 Example 19 IB 3 1996C 10 Comparative MEHQ 10 10 Example 11 Comparative HQ 10 66 Example 12Comparative AF 10 46 Example 13 Comparative 6C 10 10 Example 14

It can be seen from Table 5 that the polymerization-inhibiting effect ofthe N-oxyl compounds of the above formula (1) was further enhanced bythe combined use of other general-purpose polymerization inhibitors.

EXAMPLE 20

Using a reflux apparatus equipped with a 20-plates Oldershaw column asthe distillation column, a 3-liter four-necked flask with a side arm wascharged with 2,002 g (20 moles) of methyl methacrylate, 248 g (4 moles)of ethylene glycol, 14.4 g of dibutyltin oxide, and 0.79 g(corresponding to 0.1% of the theoretical yield of the product) of IB asa polymerization inhibitor. This mixture was subjected to an esterexchange reaction. The methanol formed as a by-product during reactionwas removed from the system together with the methyl methacrylate. Theresulting reaction product was purified by simple distillation using anempty column having a length of 20 cm, so that 530 g of ethylene glycoldimethacrylate was obtained. During the course of the reaction anddistillation, polymerization occurred neither in the reactor nor thecolumn. The ethylene glycol dimethacrylate thus obtained had a colornumber (APHA) of not greater than 5.

COMPARATIVE EXAMPLE 15

Ethylene glycol dimethacrylate was synthesized in the same manner as inExample 20, except that the polymerization inhibitor used was changedfrom IB to ID. During the course of the reaction and distillation,polymerization occurred neither in the reactor nor the column. However,the ethylene glycol dimethacrylate thus obtained had a color number(APHA) of 35.

COMPARATIVE EXAMPLE 16

Ethylene glycol dimethacrylate was synthesized in the same manner as inExample 20, except that the polymerization inhibitor used was changedfrom IB to IH. During the course of the reaction and distillation,polymerization occurred neither in the reactor nor the column. However,the ethylene glycol dimethacrylate product had a color number (APHA) of20.

COMPARATIVE EXAMPLE 17

Ethylene glycol dimethacrylate was synthesized in the same manner as inExample 20, except that the polymerization inhibitor used was changedfrom IB to AF. However, polymerization occurred in the reactor duringthe course of the distillation step.

EXAMPLE 21

Using a reflux apparatus equipped with a 20-plates Oldershaw column asthe distillation column, a 3-liter four-necked flask with a side arm wascharged with 1,401 g (14 moles) of methyl methacrylate, 701 g (7 moles)of cyclohexanol, 12.6 g of dibutyltin oxide, and 1.18 g (correspondingto 0.1% of the theoretical yield of the product) of IB as apolymerization inhibitor. This mixture was subjected to an esterexchange reaction. The methanol formed as a by-product during reactionwas removed from the system together with the methyl methacrylate. Theresulting reaction product was purified by simple distillation using anempty column having a length of 20 cm, so that 740 g of cyclohexylmethacrylate was obtained. During the course of the reaction anddistillation, polymerization occurred neither in the reactor nor thecolumn. The cyclohexyl methacrylate thus obtained had a color number(APHA) of not greater than 5.

COMPARATIVE EXAMPLE 18

Using a reflux apparatus equipped with a 20-plates Oldershaw column asthe distillation column, a 3-liter four-necked flask with a side arm wascharged with 1,401 g (14 moles) of methyl methacrylate, 701 g (7 moles)of cyclohexanol, 12.6 g of dibutyltin oxide, and 1.18 g (correspondingto 0.1% of the theoretical yield of the product) of MTX as apolymerization inhibitor. This mixture was subjected to an esterexchange reaction. The methanol formed as a by-product during reactionwas removed from the system together with the methyl methacrylate. Theresulting reaction product was purified by simple distillation using anempty column having a length of 20 cm, so that 848 g of cyclohexylmethacrylate was obtained. During the course of the reaction anddistillation, polymerization occurred neither in the reactor nor thecolumn. However, the cyclohexyl methacrylate thus obtained was slightlycolored and had a color number (APHA) of 140.

EXPLOITABILITY IN INDUSTRY

When the N-oxyl compounds represented by the above formula (1) are usedas polymerization inhibitors by adding them to vinyl monomers in smallamounts, a satisfactory polymerization-inhibiting effect is obtained athigh temperatures. Moreover, when they are used as process inhibitors,especially in the purification of α,β-unsaturated carboxylic acids andesters thereof by distillation, they are not distilled out together withthe product and hence cause no coloration thereof. Thus, they can beused as process inhibitors for vinyl monomers having high boilingpoints, without causing any trouble. This contributes greatly to thestable production of vinyl monomers, particularly α,β-unsaturatedcarboxylic acids and esters thereof.

We claim:
 1. An N-oxyl compound of the following formula (1):

wherein n is an integer of 2 to 9; R¹ and R² are each hydrogen ormethyl, but at least one of them is hydrogen; R³, R⁴, R⁵ and R⁶ are eacha straight-chain or branched alkyl group; and R⁷ is hydrogen or(meth)acryloyl.
 2. A mixture of N-oxyl compounds which comprises two ormore N-oxyl compounds of the following formula (1) having differentvalues of n:

wherein n is an integer of 1 to 18; R¹ and R² are each hydrogen ormethyl, but at least one of them is hydrogen; R³, R⁴, R⁵ and R⁶ are eacha straight-chain or branched alkyl group; and R⁷ is hydrogen or(meth)acryloyl.
 3. A process for the preparation of an N-oxyl compoundas claimed in claim 1 which comprises effecting the addition of ethyleneoxide and/or propylene oxide to a4-hydroxy-2,2,6,6-tetraalkylpiperidine-N-oxyl, and optionallyesterifying the resulting product with a (meth)acryloyl-containingcompound.
 4. A polymerization inhibition method wherein an N-oxylcompound as claimed in claim 1 is used as a polymerization inhibitor forvinyl monomers.
 5. The polymerization inhibition method as claimed inclaim 4 wherein the vinyl monomers are α,β-unsaturated carboxylic acidsand esters thereof.
 6. A polymerization inhibition method wherein theN-oxyl compound prepared according to the process of claim 3 is used asa polymerization inhibitor for vinyl monomers.
 7. A polymerizationinhibition method as claimed in claim 6 wherein the vinyl monomers areα,β-unsaturated carboxylic acids and esters thereof.
 8. A process forthe preparation of the mixture of N-oxyl compounds as claimed in claim2, which comprises effecting the addition of ethylene oxide and/orpropylene oxide to a 4-hydroxy-2,2,6,6-tetraalkylpiperidine-N-oxyl, andoptionally esterifying the resulting product with a(meth)acryloyl-containing compound.
 9. A polymerization inhibitionmethod wherein the mixture of N-oxyl compounds as claimed in claim 2 isused as a polymerization inhibitor for vinyl monomers.
 10. Thepolymerization inhibition method as claimed in claim 9, wherein thevinyl monomers are α,β-unsaturated carboxylic acids and esters thereof.11. A polymerization inhibition method wherein the mixture of N-oxylcompounds prepared according to the process of claim 8 is used as apolymerization inhibitor for vinyl monomers.
 12. The polymerizationinhibition method as claimed in claim 11, wherein the vinyl